Add PayUplift model and multiply pay periods by pay uplift

[CaitBarnard]

Add model for PayUplift
Multiply pay periods by pay uplift so that it is reflected on the forecast table in Edit Payroll
E.g:

# Person is paid Apr to Dec but not Jan to Mar
periods = [True, True, True, True, True, True, True, True, True, False, False, False]

# Uplift of 5% starts in Oct
pay_uplift = [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.05, 1.05, 1.05, 1.05, 1.05, 1.05]

# Person's pay is increased by the uplift for Oct, Nov and Dec
periods * pay_uplift = [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.05, 1.05, 1.05, 0, 0, 0]

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